(a) What was the basis of Mendeleev’s classification of elements?

(b) List two achievements of Mendeleev’s Periodic table.

(c) List any two observations which posed a challenge to Mendeleev’s periodic law.




Mendeleev classified elements according to their atomic masses .


  • Mendeleev was able to classify all the 63 elements that had been discovered at that time.
  • He left gaps for the elements yet to be discovered and also predicted their properties.


  • Since Mendeleev arranged the elements according to their atomic masses, the position of isotopes (which have the same mass number but different atomic number) could not be decided.
  • The position of Hydrogen could not be decided.
  • Since the atomic mass increases irregularly from one element to the next, this makes it difficult to predict the position of undiscovered elements.
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CA Maninder Singh is a Chartered Accountant for the past 14 years and a teacher from the past 18 years. He teaches Science, Economics, Accounting and English at Teachoo